When combustion is performed in a cylinder, the angular velocity of a crankshaft is increased from a first angular velocity ωa to a second angular velocity cob by combustion pressure. Therefore, when I represents the rotational moment of inertia of an engine, kinetic energy E increases from (½)·Iωa2 to (½)·Iωb2. In other words, torque is produced according to an increasing amount of this kinetic energy, ΔE=(½)·I·(ωb2−ωa2), and hence the produced torque is proportional to (ωb2−ωa2).
In this manner, the produced torque is obtained from the difference between the square of the first angular velocity ωa and the square of the second angular velocity ωb. Therefore, in an engine control method, for example as disclosed in Patent Literature 1, torque variation is calculated based on the produced torque obtained from the difference between the square of the first angular velocity ωa and the square of the second angular velocity ωb.